1. Field of the Invention
The present invention relates to an electric power steering apparatus which relieves the steering force of a driver by causing the driving force of an electric motor to directly act on a steering system, thereby, and more particularly to an electric power steering apparatus capable of properly carrying out unload control.
2. Description of the Related Art
An electric power steering apparatus serves to relieve (assist) the steering force of a driver by causing the driving force of an electric motor to directly act. A vehicle having the electric power steering apparatus mounted thereon has generally been spread widely. By the electric power steering apparatus, the movement of a steering wheel becomes quick and the driver does not need to carry out a steering operation by strong force.
The electric power steering apparatus includes an electric motor driving circuit constituted by a bridge circuit of a field effect transistor (hereinafter referred to as an xe2x80x9cFETxe2x80x9d) for generating an electric motor driving signal to drive an electric motor in response to an electric motor control signal (a control signal) output from a controller. The FET generates heat when producing the electric motor driving signal based on the control signal. When the heat generation is excessively maintained, the temperature of the FET is largely raised, so that the function of the FET is deteriorated or the FET is broken in some cases. In order to prevent the deterioration or the breakage, accordingly, unload control (the limitation of the maximum value of the control signal) is carried out, in which the maximum value of the control signal is limited when the temperature of the FET is raised, so that the heat generation of the FET is controlled to drop the temperature of the FET.
In order to carry out the unload control, it is considered that a temperature sensor for measuring the temperature of the FET is used. However, it is not preferable that the temperature sensor should be used because the cost of the device is increased. For this reason, the temperature of the FET is estimated without using the temperature sensor and the unload control is carried out based on the estimated temperature. Conventionally, the temperature of the FET has been estimated by the following Equation 1.
Temperature of FET ≈
Atmospheric temperature (initial set value)+∫ (current value in predetermined time)xe2x80x83xe2x80x83(Equation 1)
The atmospheric temperature is an initial set value (fixed value) for estimating the temperature of the FET. The atmospheric temperature is set to a safety side in order to reliably prevent the heating failure of the FET (to prevent the FET from being damaged on the worst conditions). Usually, the electric power steering apparatus is attached to an engine room provided in front of a vehicle. Therefore, the atmospheric temperature is set to a high temperature of approximately 80xc2x0 C. on the basis of the temperature of the engine room. A current value in a predetermined time (a current value of an electric motor current) is integrated and added to estimate the temperature of the FET, and the unload control is carried out based thereon. Consequently, the actual temperature of the FET does not exceed a limit temperature (for example, 120xc2x0 C.) Therefore, the FET can always be used in good conditions.
In the conventional electric power steering apparatus, however, when the temperature of the engine room is low, for example, the engine of the vehicle is just started up or when the actual temperature of the FET is low, for example, straight running is maintained for a while and the FET is not operated, if a large current flows in a predetermined time, it is decided that the temperature of the FET exceeds the limit temperature and the unload control is started. In other words, the unload control is started unnecessarily. When the unload control is started, since the maximum value of a control signal is limited, the maximum value of an electric motor driving signal supplied to the electric motor is also limited. For this reason, the driver cannot fully receive an assist through the auxiliary steering torque of the electric motor. Thus, a steering feeling is deteriorated. On the other hand, when the atmospheric temperature in the Equation 1 is set to be low, the unload control is not started when it is necessary and the function of the FET is deteriorated or the FET is damaged due to heat generation.
Therefore, it is an object of the invention to provide an electric power steering apparatus capable of properly carrying out unload control.
In consideration of the problems, the inventors made a study eagerly and paid attention to the fact that the actual temperature of the FET obtained when the maximum value limitation of a control signal is started through unload control at a first time is lower than the actual temperature of the FET obtained when the maximum value limitation is started at a second time and thereafter, the maximum value limitation for the first time is not actually necessary or the maximum value limitation may be less carried out than that for the second time and thereafter. Thus, they completed the invention.
More specifically, in order to solve the problems, a first aspect of the invention is directed to an electric power steering apparatus comprising an electric motor for adding an auxiliary steering torque to a steering system of a vehicle, a steering torque detecting unit for detecting a manual steering torque of the steering system, a control unit for outputting a control signal controlling the electric motor in response to at least a signal sent from the steering torque detecting unit, a driving control unit including a switching element for driving the electric motor in response to the control signal sent from the control unit, and a current detecting unit for detecting an electric motor current flowing to the electric motor, the control unit controlling so as to limit a maximum value of the control signal in response to a signal sent from the current detecting unit. The control unit includes a limitation number measuring unit for measuring the number of times of a maximum value limitation of the control signal and has such a structure that a condition for starting the maximum value limitation of the control signal are set to be more strict at a first time than that at a second time and thereafter.
The maximum value of the control signal (electric motor control signal) is limited in response to the signal (electric motor current signal) sent from the electric motor current detecting unit and the unload control is started. With such a structure, the conditions (maximum value limitation start conditions) for starting the maximum value limitation of the control signal is caused to be strict at the first time. The wording xe2x80x9cstrictxe2x80x9d means that the maximum value limitation at the first time is started more difficult than that at the second time and thereafter. In other words, the temperature of the FET obtained when the maximum value limitation is started at the first time is lower than the temperature of the FET obtained when the maximum value limitation is started at the second time and thereafter. Therefore, such a maximum value limitation is carried out. Corresponding to the difficult start of the maximum value limitation, a driver is assisted in a steering operation through the auxiliary steering torque of the electric motor. The maximum value limitation start conditions can be set variously. When the maximum value limitation of the control signal is carried out, a load on the FET is decreased so that the temperature of the FET is reduced. While the engine is started up and is then stopped, the maximum value limitation to be carried out at the first time is not always performed once but plural times in some cases. The reason is that the temperature of the FET is reduced and the maximum value control to be carried out at the second time may be set to be performed at the first time. In this respect, the invention has the same situation.
Moreover, a second aspect of the invention is directed to an electric power steering apparatus comprising an electric motor for adding an auxiliary steering torque to a steering system of a vehicle, a steering torque detecting unit for detecting a manual steering torque of the steering system, a control unit for outputting a control signal controlling the electric motor in response to at least a signal sent from the steering torque detecting unit, a driving control unit including a switching element for driving the electric motor in response to the control signal sent from the control unit, and a current detecting unit for detecting an electric motor current flowing to the electric motor, the control unit controlling so as to limit a maximum value of the control signal in response to a signal sent from the current detecting unit. The control unit includes a limitation number measuring unit for measuring the number of times of a maximum value limitation of the control signal and has such a structure that the maximum value limitation of the control signal is set to be smaller at a first time than that at a second time and thereafter.
Also in the invention, the maximum value of the control signal is limited and the unload control is started in response to the signal sent from the electric motor current detecting unit. With such a structure, the maximum value limitation of the control signal is started on the same maximum value limitation start conditions at the first time and the second time and thereafter, for example. However, the contents of the maximum value limitation are varied, the maximum value limitation is more reduced (the maximum value limitation is more relieved) at the first time and a large auxiliary steering torque can be generated. The temperature of the FET obtained when the maximum value limitation is to be started at the first time is lower than the temperature of the FET obtained when the maximum value limitation is started at the second time and thereafter. Therefore, such a maximum value limitation is carried out. The maximum value control start conditions may be varied.